The invention pertains to the field of arc fault detectors and interrupters, and in particular, to an arc fault interrupter which detects and interrupts line side series or parallel arcing.
A percentage of fires each year are caused by electrical branch circuit wiring arcing faults involving currents below the trip level of a conventional circuit breaker or OCPD (over current protection device) as well as below the handling rate of the breaker. Basic overcurrent protection afforded by circuit breakers is designed to prevent I2R heating of the wiring in the electrical distribution system, caused by circuit overloading or line-to-line faults, and not necessarily arcing faults. A true short circuit is a rarity in an electrical system. In fact, it is more accurate to think of electrical faults as having some level of impedance, such as a high impedance arc fault (low current) or a low impedance fault (high current). Many electrical faults begin as high impedance breakdowns between the line and neutral conductors or to the ground wire or device components. AFCI (Arc Fault Circuit Interrupter) technology affords protection from conditions that may not necessarily be an immediate threat but could become hazardous if left unattended.
In order to start a fire, three elements must be present fuel, oxygen (air), and energy to ignite the fuel. Arcing is defined as a luminous discharge of electricity across an insulating medium. The electrical discharge of an arc can reach temperatures of several thousand degrees Celsius. Arcing produces sufficient energy to reach the ignition point of nearby combustible material(s) before a circuit breaker can respond. Arc detection is an enhancement to thermal magnetic overload detection typically used in circuit breakers or OCPD""s, which alone may not detect and respond to arc faults.
A number of devices for detecting arc faults and methods of detection have been used in the past. These include using E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, using non-overlapping band pass filters to detect white noise characteristic of arcs, and utilizing the high frequency components (RF) of arcing waveforms to detect arcing faults. While some of these techniques are more or less effective than others, they require relatively sophisticated arc sensors and circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers.
xe2x80x9cA-typexe2x80x9d arc faults are those in which the arc occurs across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network. The conductors are carrying current to a load derived from the line voltage. The arc could likewise occur as a break or at a loose terminal associated with an extension cord deriving power from line voltage, thereby completing the circuit to the load. Since the current through the A-type fault is limited by the impedance of the load itself, since the fault is in series with the load, an A-type fault is also known as a xe2x80x9cseries fault.xe2x80x9d
xe2x80x9cB-typexe2x80x9d arc faults are a second arcing condition that must be detected and interrupted by a combination outlet device. In a B-type fault, the arc occurs across two conductors in the branch circuit or extension cords plugged into it, at a site where the insulating media separating the two conductors has been compromised. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse-polarized, between the neutral and ground conductors. The current through the B-type fault is not limited by the impedance of the load, but rather by the available current from the supply established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, i.e., the conductive members carrying the fault current. Since B-type faults are effectively across the line, they are also known as xe2x80x9cparallel faults.xe2x80x9d
There is a need for simple economical arc fault detectors that can be included in wiring devices such as duplex receptacles, multi-outlet strips, or in-line devices, and that offer the same protection as an arc fault detector incorporated in a circuit breaker but at lower cost. There is a need for an arc fault circuit detector in wiring devices that can be provided at a reduced cost compared with arc fault circuit detecting circuit breakers comparable to the reduction in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers.
Briefly stated, An arc fault causes the line voltage across the line terminals of an arc fault circuit interrupter (AFCI) to change its characteristic voltage pulse shape as the line voltage is momentarily removed from the AFCI terminals after the arc extinguishes and before it re-strikes by introducing a flat voltage portion to the pulse shape. This flat voltage portion changes the voltage pulse width. An arc detector/processor detects this change in pulse width to produce a signal indicative of upstream (line side) arcing. The flat voltage portion can also be detected using clamping diodes and charging capacitors.
According to an embodiment of the invention, an arc fault detector operatively connected to first and second lines of an AC electric power distribution system includes first means for determining a first width of a voltage pulse of a line voltage across the first and second lines in an absence of an arc fault on a line side of the arc fault detector; second means for determining a second width of a voltage pulse of the line voltage across the first and second lines in a presence of the arc fault on the line side of the arc fault detector; comparison means for comparing the first width to the second width; and means for producing a signal when the first width exceeds the second width for a predetermined number of line cycles of the line voltage.
According to an embodiment of the invention, an arc fault detector operatively connected to first and second lines of an AC electric power distribution system includes means for sampling a voltage pulse of a line voltage across the first and second lines on a line side of the arc fault detector; and means for determining when the voltage pulse changes from a normal sine wave to an abnormal sine wave characterized by a flat voltage region, thereby indicating an arc fault.
According to an embodiment of the invention, an arc fault detector operatively connected to first and second lines of an AC electric power distribution system includes means for determining a width of a voltage pulse of a line voltage across the first and second lines in a presence of an arc fault on a line side of the arc fault detector; comparison means for comparing the width to a width constant held in a memory; and means for producing a signal when the width exceeds the width constant.
According to an embodiment of the invention, an arc fault detector operatively connected to first and second lines of an AC electric power distribution system includes first means for detecting a shift from a sine waveform to a flat top waveform which has a voltage level between two predetermined levels for a predetermined interval during a half wave; second means for detecting the shift for a predetermined number of half waves; and means, responsive to the first and second means, for producing a signal.